Robots could flex muscles that are stronger than steel

A metre long ribbon of a carbon nanotube ‘aerogel’ that could make a robust artificial muscle. This ribbon more than trebles its width when a voltage is applied

(Image: Ray Baughman)

A new material that is weight for weight stronger than steel and stiffer than diamond, and weighs little more than its volume in air, could be the perfect artificial muscle for robots.

“We’ve made a totally new type of artificial muscle that is able to provide performance characteristics that have not previously been obtained,” says Ray Baughman, a materials scientist at the University of Texas, Dallas, and co-developer of the new muscle.

Baughman and colleagues have developed a technique to make ribbons of tangled nanotubes that expand in width by 220% when a voltage is applied and then return to their normal size once it is removed. The process takes only milliseconds.

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Tough workers

Collections of those ribbons could act as artificial muscle fibres – for example, to move the limbs of a walking robot, says Baughman. And the material has other impressive properties.

It is extremely stiff and strong in the “long” direction – that in which the nanotubes are aligned – but is as stretchy as rubber across its width. It also maintains its properties over an extreme range of temperatures&colon; from -196 °C, at which temperature nitrogen is liquid, to 1538 °C, above the melting point of iron.

This means any robot equipped with the nanotube muscles could potentially keep working in some very extreme environments.

The new material has some advantages over previous artificial muscles. Some of those work only when bathed in methanol fuel, others are capable of only very small changes in size and none of them work well at extreme temperatures.

Space construction

The tangled nanotubes are constructed into a film that can be described as an aerogel, meaning it contains more air than anything else. Each cubic centimetre weighs only 1.5 milligrams and, given the film’s thinness, a single gram would cover 30 square metres.

Ribbons of the aerogel are made by first growing “forests” of carbon nanotubes that resemble a dense thicket of bamboo stalks. The researchers then stick a length of adhesive to the sides of those stalks and pull gently to draw out a long, thin film of the tubes, which tangle during the process. So far, ribbons a 50th of a millimetre thick by 16 centimetres wide and and several metres long have been made, but it should be possible to form larger sheets by starting with more nanotubes.

Electrical engineer John Madden at the University of British Columbia, who was not involved in developing the material, says that resilience and low density could make it a good material for building structures in space – its lightness keeping down the cost of sending a payload into orbit.

However, the low density could be a problem, says Boris Yakobson, a materials scientist at Rice University in Houston. The new material is roughly as strong as human muscle by weight, but, because it is 1000 times less dense, a large volume would be needed to match the strength of, say, a human arm.

Research should focus on making a denser material with the same properties, he says. “What they present sounds great and motivating. Yet, I would say, for applications it still needs a lot of improvement.”